Echo cancelers are used in communication devices such as hands-free automobile telephone sets. An echo canceler has a filter that computes a signal received from a far-end communication device, estimates the echo that the received signal will produce in the local input signal, and creates a replica of the estimated echo. The echo replica is subtracted from the local input signal so that the party at the far end will not hear an echo of his or her own voice.
The filter has tap coefficients that represent the transfer characteristics of the echo path. To deal with changes in the characteristics of the echo path, and with external noise, the tap coefficients are adjusted by an algorithm that attempts to reduce the residual echo left after echo cancellation. Two widely-used algorithms are the least mean squares (LMS) algorithm and the normalized least mean squares (NLMS) algorithm, both of which have the advantages of stable operation and comparatively light computational requirements.
The conventional echo canceler has the disadvantage, however, of extensive computation. According to the using environment, the echo path becomes long. When the echo path is created without decline of performance to cancel echo components, an adaptive filter requires more taps, and computational requirements become extensive. According to increase of computational requirements, more memory which store a signal received from a far-end communication device is required, and computational requirements of the adaptive filter become extensive. This is a disadvantage without regard to adaptive algorithm.
In an automobile telephone using the echo canceler, for example, the twice taps of a short term echo path are required to cancel echo components with the same accuracy as canceling short term echo components. At least, the twice computational requirements of canceling short term echo components is required to create the echo replica by the sample value of a signal received from a far-end communication device. This is the cause of the increase of computational requirements in the whole device. The echo canceler that can deal with the longer impulse response by setting the bit allocation of tap coefficients of adaptive filter have been proposed in echo canceler algorithm.